Polysialylation in eukaryotes is catalyzed by sialyltransferases of the ST8Sia family that attach negatively charged sialic acid sugars onto glycoprotein and -lipid acceptors. We solved the crystal structure of human ST8SiaIII at 1.85 Å resolution and investigated structural motifs that provide an extended electropositive groove for binding of oligo-polysialic acid chain products and acceptor proteins. A sialyltransferase glycan array helped us to identify a novel acceptor sugar which we subsequently used to obtain a ternary complex and to characterize substrate binding, specificity and sialyl transfer.